Five hundred Oregon silverspot butterfly caterpillars have been released on the slopes of Saddle Mountain, part of a continuing effort to stabilize the declining species population in the state.

The reintroduction was led by a team from Oregon Parks and Recreation Department (OPRD), U.S. Fish and Wildlife Service (USFWS), Oregon Zoo and Woodland Park Zoo in Seattle.

The Oregon silverspot butterfly (Speyeria zerene hippolyta) was federally listed as threatened in 1980, and population numbers have declined continuously over the last three decades. Today, just four isolated populations remain: three in Oregon and one in California.

Saddle Mountain was chosen as the reintroduction site because a rare flower—the early blue violet—blooms in abundance there. Early blue violets are the main food source for the caterpillars as they mature into adult butterflies. The mountain is one of the few remaining areas where early blue violets grow in large enough quantities to sustain a butterfly population. Elsewhere, the delicate violets have been choked out by invasive weeds and forest succession.

“Saddle Mountain is prime real estate for Oregon silverspots,” said Trevor Taylor, manager for the reintroduction project at OPRD. “Our hope is the caterpillars will be the start of a vibrant and lasting butterfly population on the mountain.”

The caterpillars began their journey to the mountain as part of the imperiled species programs at Oregon Zoo and Woodland Park Zoo. Each year, a small number of female Oregon silverspots are collected from wild populations and brought to zoo conservation labs to lay eggs. The hatched larvae are raised over the winter and released into the wild when they’ve matured into caterpillars.

Funding for the reintroduction project was provided by the USFWS Cooperative Endangered Species Conservation Fund.